1). Field of the Invention
This invention relates to a semiconductor chip, a semiconductor package, to a method of assembling a semiconductor package, and to a method of producing a semiconductor chip.
2). Discussion of Related Art
The term "frontside" of a semiconductor wafer or semiconductor chip, as used herein, denotes the side of the semiconductor wafer or semiconductor chip which carries an integrated circuit, and the term "backside", as used herein, denotes a side of the semiconductor wafer or the semiconductor chip opposing the frontside.
A semiconductor package usually includes a package substrate and a semiconductor chip located on the package substrate. A semiconductor chip is often provided with C4 (controlled collapse chip connect) solder connections, on a frontside thereof, for purposes of electrically contacting an integrated circuit on the frontside of the chip to contact pads on the package substrate. An epoxy is typically introduced under capillary action into a space provided between the semiconductor chip and the package substrate and is cured. The epoxy acts to bond the semiconductor chip to the package substrate and to protect the fragile C4 solder connections.
The semiconductor chip is made primarily of silicon which has a coefficient of thermal expansion (CTE) of about 3.3 ppm/.degree. C. (parts per million per .degree. C.). In the past, the package substrate was generally made of a ceramic material, which has a coefficient of thermal expansion that is typically below 6 ppm/.degree. C. During heating or cooling of the semiconductor package the coefficients of thermal expansion of the semiconductor chip and the ceramic package substrate, respectively, were not of a magnitude which caused substantial bending of the semiconductor chip.
Ceramic has a relatively high dielectric constant which causes stray capacitance to build up within the package substrate, resulting in resistance-capacitance (RC) delay. The move in recent years has therefore been away from ceramic as a package substrate material to alternative materials, such as plastics or other organic materials, which have lower dielectric constants. A problem with these alternative materials is that they usually have relatively high coefficients of thermal expansion, compared to the coefficient of thermal expansion of the semiconductor chip. Some plastics materials, for example, have coefficients of thermal expansion of the order of 17 ppm/.degree. C. Heating or cooling of the semiconductor package thus results in substantial bending of the semiconductor chip.
As discussed above and with reference to FIG. 1, an epoxy material 10, generally a glass-filled epoxy, is provided within the space between the semiconductor chip 12 and the package substrate 14 and cured. The step of curing the epoxy involves elevating the temperature of the semiconductor package 16 to a given temperature for a specific period of time. Once the curing procedure is complete, the semiconductor package is then cooled to ambient temperature. FIG. 1 illustrates an organic semiconductor package 16 after being cured and cooled to ambient temperature. Since the coefficient of thermal expansion of the organic package substrate 14 is much greater than the coefficient of thermal expansion of the semiconductor chip 12, the package substrate 14 tends to reduce in size during cooling at a much faster rate than the semiconductor chip 12. This causes the semiconductor package 16 to warp in a manner that results in outward bowing or bending of the semiconductor chip 12.
Bending or bowing of the semiconductor chip is problematic, in that it induces greater stresses along the backside 18 of the semiconductor chip. Surface defects 20, such as nicks and scratches, are generally present along the backside 18 of the semiconductor chip as a result of the process and handling procedures used during the manufacturing process. The bending or bowing of the chip can cause the surface defects to develop rapidly into larger cracks which propagate through the semiconductor chip. Propagation of the defects can cause severe damage to the chip, and can eventually cut through active circuitry 21 on a frontside 22 of the semiconductor chip, resulting in electrical failure of the semiconductor chip.